Process for the recovery of a polyol from an aqueous solution

ABSTRACT

A process for the separation of a polyol or multiple polyols in admixture with other organic compounds, usually those produced with the polyol, is described. The process uses a distillation in a column ( 11 ) of a cyclic acetal from an aqueous solution which acetal is formed in a reaction mixture of the polyol and an aldehyde or ketone. The polyols, such as ethylene glycol and propylene glycol, are staple articles of commerce with many uses.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of application Ser. No. 09/891,955filed Jun. 26, 2001 now U.S. Pat. No. 6,548,681.

STATEMENT REGARDING FEDERALLY FUNDED RESEARCH OR DEVELOPMENT

None

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a process for the separation of apolyol from an aqueous solution. The process involves reactivedistillation of the polyol as a cyclic acetal from an aqueous reactionmixture containing other organic compounds, particularly other polyols.In particular, the cyclic acetal is prepared by reaction of a ketone oraldehyde with the polyol along with distillation of cyclic acetal as itis formed from the reaction mixture.

(2) Description of the Related Art

There is a need to recover and purify polyols, including glycols, froman aqueous solution. These polyhydroxy compounds are typically formed inmultistep processes in dilute aqueous solutions, from which thepolyol(s) must be separated and purified before being used or sold.These processes include, but are not limited to, production of ethyleneglycol and propylene glycol from their respective epoxides, formation ofpropylene glycol from glycerol, and formation of polyols viahydrogenolysis of sugars or sugar alcohols. All of these processesproduce dilute mixtures of organic compounds including the desiredpolyols in the aqueous solution.

In the presence of acidic catalysts, glycols (or other polyols) reactreversibly with aldehydes and ketones to form cyclic acetals. Thereaction is known as acetalization or ketalization. The acetals of thepolyols are far more volatile than the polyols themselves and much lesspolar, making them easily separable from water by distillation. Becausethe acetalization reaction is reversible, glycols and the aldehyde canbe regenerated by acid hydrolysis of the acetal. The glycol can then berecovered and the aldehyde can be recycled. Ion exchange resins (IER)are one class of materials that can effectively catalyze acetalformation and hydrolysis, but mineral acids and other solid acids areeffective as well. The reaction is as follows:

Of general interest in connection with this type of reaction in anon-cyclic context is Mahajani, S. M. et al., Reactive And FunctionalPolymers 28 29-38 (1995).

There have been several reports of the reaction of glycols withaldehydes to form cyclic acetals. Tink and coworkers (Tink, R. R., etal., Can. J. Technol., 29, 243 (1951)) have published a series of papersdescribing recovery of aqueous glycerol solution via reactive extractionwith various aldehydes. As disclosed, n-butyraldehyde and cyclohexanonewere promising among the several aldehydes studied and the former wasparticularly selective. They also studied reactive extraction of severalpolyhydroxy compounds including D-sorbitol, adonitol, dulcitol,D-mannitol and ethylene glycol from aqueous solutions. High distributioncoefficients were obtained with reactive extraction. For instance, withn-butyraldehyde the distribution coefficient for glycerol is 8.3, for EGis 5.9 and for D-sorbitol is 788.

Broekhuis et al. (Broekhuis, R. R., et al., Ind. Eng. Chem. Res., 33,3230 (1994)) have compared the various routes for the recovery ofpropylene glycol from dilute aqueous solutions via reaction withaldehydes. They studied lower aldehydes, viz. formaldehyde andacetaldehyde, for reactive distillation and extractive reaction for therecovery. They have claimed to achieve 99+% recovery of propylene glycolin a reactive distillation process. One of the present inventors hasreported on the recovery of ethylene glycol from aqueous solution viaacetalization with formaldehyde (Chopade, S. P. and Sharma, M. M., ReactFunct. Polym. 34(1) 37 (1997)) using ion exchange resins as catalysts.

A search of the patent literature reveals no processes combiningacetalization with reactive distillation of cyclic acetals for polyolseparation. U.S. Pat. No. 5,917,059 to Bruchmann et al. describespreparation of the cyclic acetal compounds, but does not discuss them incontext of a separation scheme for glycol recovery. There are numerouspatents that describe inventions pertaining to acetals, acetalization,and reactive distillation, but none were found that pertained to ascheme for the recovery of polyols, especially from a dilute mixedsolution of polyols, such as a sugar hydrogenolysis effluent.

Polyhydroxy compounds show a high affinity towards water and each otherbecause of hydrogen bonding, and separation of these products fromaqueous solution is conventionally done via a multi-column distillationprocess. In order to obtain ethylene glycol (EG) and propylene glycol(PG), water must be distilled off first because it has a lower boilingpoint temperature than the polyols. The energy to distill off water isthe primary reason for the high cost of polyol separation and recovery.Separation of EG and PG (if they are present together) is also costlybecause they have very similar boiling points, so that a large number ofstages and a large reflux ratio, translating to a large distillationcolumn, is required to achieve the required purities. Purification ofglycerol in a simple distillation column without forming poly-glyceridesand decomposition products is impossible. Vacuum distillation, which hashigh operating costs, is the only distillation route for direct glycerolrecovery.

Another approach for polyol recovery is solvent extraction of polyolsfrom water. Glycols and glycerol have high affinity towards water (againbecause of hydrogen bonding), and it is difficult to find a suitablesolvent with good distribution coefficient and low miscibility withwater. Further, extraction only eliminates distillation of large amountsof water from the product stream. After extraction, there aredistillation steps involving solvent recovery followed by separation ofpolyols from each other. Thus extraction is similar to distillation,except that water is replaced by a solvent.

There is a need for a safe and effective process for the production ofpolyols. In particular there is a need for a process to efficientlyseparate EG and PG from aqueous solutions.

OBJECTS

It is therefore an object of the present invention to provide aneconomical and efficient process for the separation of at least onepolyol from water. It is further an object of the present invention toprovide a process which is relatively easy to perform on a large scalesuitable for commercial production of polyols such as EG and PG. Theseand other objects will become increasingly apparent by reference to thefollowing description and the drawings.

SUMMARY OF THE INVENTION

The present invention relates to a continuous process for preparing atleast one acetal from an aqueous solution containing at least one polyoland at least one other organic compound which comprises:

(a) reacting in a combination reaction and distillation vessel areaction mixture of the polyol and an aldehyde or ketone containing 1 to4 carbon atoms in the aqueous solution in the presence of an acidcatalyst, wherein the reaction mixture is introduced into the reactionvessel containing the catalyst with a molar excess of the aldehyde orketone over the polyol to produce the cyclic acetal in the aqueoussolution; and

(b) separating at least one cyclic acetal from the reaction mixture bydistillation.

Further, the present invention relates to a continuous process forrecovering a polyol from an aqueous solution containing other organiccompounds which comprises:

(a) reacting in a combination reaction and distillation vessel areaction mixture of the polyol and an aldehyde or ketone containing 1 to4 carbon atoms in the aqueous solution, wherein the reaction mixture iscontinuously introduced into the vessel containing the catalyst with amolar excess of the aldehyde or ketone over the polyol to produce acyclic acetal in the aqueous solution;

(b) separating the acetal from the mixture at elevated temperatures; and

(c) hydrolyzing the cyclic acetal produced to recover the polyol as aliquid and the acetaldehyde or ketone which is separated as a vapor fromthe polyol.

Preferably the reaction mixture is at a temperature, less than theboiling point of the reaction mixture, at which at least the aldehyde orketone is distilled from the reaction vessel as a distillate. Also,preferably if there is more than one cyclic acetal produced, the cyclicacetals are separated before the hydrolysis step is performed. Theseparation can be accomplished in the reaction vessel for the reactivedistillation or in a separate vessel connected to the reaction vessel.Typically the reaction vessel(s) is a heated column. Preferably thedesired cyclic acetal is also distilled from the reaction vessel andseparated from the aldehyde or ketone.

Preferably the reaction mixture is at a temperature, less than theboiling point of the reaction mixture, at which at least the aldehyde orketone is distilled from the aqueous solution as a distillate.Preferably the ketone or aldehyde is recycled.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic front view of a polyol recovery system 10 with asingle reactive distillation column 11 for forming the cyclic acetal(s).EG is ethylene glycol, PG is propylene glycol, and Gly is glycerol.

FIG. 2 is a schematic front view of the polyol recovery system 10including reactive distillation column 11 and acetal hydrolysis andpolyol recovery columns 18 and 19. MD is 2-methyl-1,3-dioxolane (acetalof EG), DMD is 2,4-dimethyl-1,3-dioxolane (acetal of PG).

FIG. 3 is a schematic view of reactive distillation column 21 and acetalhydrolysis and polyol recovery columns 25 and 26 as an alternative tothe process of FIG. 2 for the recovery of EG and PG.

FIG. 4 shows the experimental column for Example 2.

FIG. 5 is a graph showing a binary T-x-y vapor liquid-liquid equilibrium(VLLE) diagram for 2MD and water.

FIG. 6 is a binary T-x-y VLLE diagram for 24DMD and water.

FIG. 7 is a graph showing vapor pressure data for DMD and MD at varioustemperatures. The data of FIGS. 5 to 7 was developed for the presentinvention.

FIG. 8 is a graph showing a plot of relative volatility of MD/DMD vstemperature.

FIG. 9 is a process schematic showing two columns 100 and 101 for acetalformation in column 100 and for acetal recovery in column 101. Thecolumn 101 for recycle of acetaldehyde is on the right.

FIG. 10 is a schematic diagram similar to FIG. 2, except that the PG andEG are separated by distillation in column 30 after hydrolysis of themixed acetals in column 17.

FIG. 11 is an additional view of the hydrolysis portion of the process,shown as vessels 17, 18 and 19 shown in FIG. 2.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention relates to a process involving acetalization andreactive distillation of a polyol that takes advantage of the reversiblereaction of the acetal which is formed and facilitates the separationand recovery of the polyol from an aqueous solution. The focus of theinvention is on recovering ethylene and propylene glycols from aqueoussolutions containing higher polyols, but the method has broaderapplications to recovery of a wide range of polyols from water. Boilingpoints of some acetals of interest for this process are given in Table1.

TABLE 1 Boiling points of some acetals and aldehydes Glycol AldehydeAcetal b.p., ° C. EG Formaldehyde 1,3-dioxolane 74-75 PG Formaldehyde4-methyl-1,3- 84 dioxolane Glycerol Formaldehyde Glycerol formal 191-195EG Acetaldehyde 2-methyl-1,3- 82-83 dioxolane PG Acetaldehyde2,4-dimethyl-1,3- 91-93 dioxolane EG Acetone 2,2-dimethyl-1,3- 92-93dioxolane PG Acetone 2,2,4-trimethyl- 98-99 1,3-dioxolane GlycerolAcetone 188 Formaldehyde 101 Acetaldehyde 21 Isobutyr- 64 aldehydeAcetone 56

The reactive distillation process involves several distillation columnswhere the acetals are formed, separated from water, and subsequentlyhydrolyzed back to the desired polyol. The acetalization/reactivedistillation scheme facilitates polyol recovery and purification atlower cost than conventional distillation or extraction methods. For therecovery of ethylene glycol and propylene glycol, acetals formed withformaldehyde or acetaldehyde have lower boiling points than water, sothey can be removed once formed without having to boil off all the waterpresent. This offers a large cost savings over conventionaldistillation. Further, these acetals have much lower boiling points thanacetals of higher polyols such as glycerol, sorbitol and xylitol, whichcan be in potential feedstocks for EG/PG production, and acetals ofprocess byproducts such as C₄ and C₅ polyols, so they are easilyseparated from the higher polyols and their acetals in solution. Thisaspect of the invention distinguishes it from the work of Broekhuis etal (1994) and Chopade and Sharma (1997), who did not consider recoveryof EG and PG from mixed polyols. Further, the acetals of EG and PG canbe separated from each other at much lower temperatures and potentiallymore easily than EG and PG themselves, so the cost of the polyolseparation is substantially lower as well. Overall, the integration ofthe acetalization scheme into a biomass-based polyols process enhancesthe commercial usefulness of the process.

Ethylene glycol and propylene glycol are large-scale commoditychemicals: EG is produced at rate of 17 billion lb/yr and PG at about 1billion lb/yr. The present invention provides a more energy efficientroute for EG and PG recovery than the conventional distillation methods.This invention can be combined with the biomass-based production of EGand PG via sugar and sugar alcohol hydrogenolysis to provide aneconomical, renewable resource-based route to EG and PG production.

The preferred embodiment of the present invention is the recovery of EGand PG from a mixed polyols stream resulting from hydrogenolysis ofsugars or sugar alcohols to polyols. The mixed polyol stream fromhydrogenolysis contains, in addition to EG and PG, glycerol, unreactedfeed (C₅ or C₆ sugar alcohol), and other organic compound byproductssuch as C₄ polyols and lactic acid. Unreacted feed and byproducts arecollectively referred to as “other organic compounds” hereinafter.

The choice of which acetal to form, e.g. which aldehyde or ketone to useto acetalize the diols to be recovered, has a strong affect on theconfiguration of the process. Formaldehyde was used in the initialfeasability study. However, formaldehyde is a nuisance to theenvironment. Acetals can be formed with a large number of carbonylcompounds, but only acetaldehyde, formaldehyde, and acetone give acetalsof PG and EG which have boiling points below that of water. Thus thepreferred chemical for recovery of EG and PG is acetaldehyde. Theboiling point of acetaldehyde is 21° C. and hence a closed system with agood chilling facility is required. The reactions can be carried outunder pressure, if necessary, to enhance the acetaldehyde concentrationin the liquid phase.

In reactive distillation, the potential exists for multiple reactions totake place in a single distillation column or, if desired, in multipledistillation columns. The use of a single column can lead to substantialsavings in capital as well as operating cost. A schematic of a singlecolumn 11 reactive distillation process is shown in FIG. 1. FIG. 1 showsthe system 10 including the reactive distillation column 11. The column11 is divided into four sections to help understand the processconcepts. The sections are the acetalization section 13, the enrichingsection 12, the hydrolysis section 14 and the stripping section 15.Sections 12 and 15, the enriching and stripping sections, respectively,are non-reactive, and there is no catalyst in these sections. Sections13 and 14 are the reactive sections, which are the key sections in theprocess. The aqueous solution containing EG, PG, glycerol, and otherproducts is fed at the top of section 13. Acetaldehyde is fed at a pointwhere sections 13 and 14 meet. Acetaldehyde, being the most volatilecomponent, moves up the distillation column through section 13. Theaqueous feed moves down section 13 and reacts with acetaldehyde, formingacetals. The acetals of EG and PG, being more volatile than water, moveup the column into non-reactive section 12. Section 12 strips water fromthe acetals and the acetals of EG and PG exit at the top of the columnalong with unreacted acetaldehyde.

Because there is essentially no acetaldehyde in section 14, the acetalsof glycerol and other organic compounds will hydrolyze in the presenceof catalyst back to glycerol and other organic compounds andacetaldehyde. Any acetaldehyde released will quickly move up the column11 and glycerol and other products are recovered at the bottom of thecolumn 11.

FIG. 2 shows the column 11 in a reactive distillation system 10 with theseparation of MD (2-methyl-1,3-dioxolane, which is the acetal of EG) andDMD (2,4-dimethyl-1,3-dioxolane, the acetal of PG). Flash drum 16 allowsthe separation of MD and DMD from acetaldehyde which is recycled. Column17 separates 2MD from DMD. Recovery column 18 is for hydrolysis of MDand recovery column 19 is for hydrolysis of DMD. Vessel 20 is ademineralization vessel to remove inorganic compounds (salts, forinstance) if present in the feed stream. Thus, in FIG. 2, acetaldehydeis separated from the acetals in the small column or flash drum 16 andis recycled back to the acetaldehyde feed to the column 11. The acetalsare then separated from each other in the second column 17. Finally,each acetal is separately hydrolyzed back to its corresponding glycol(EG or PG) in the columns 18 and 19, and acetaldehyde is recycled backto column 11.

FIG. 3 shows an alternate scheme where the acetals are recovered at thebottom of the reactive distillation column 21. The potential advantageof this scheme is that only a stoichiometric quantity of acetaldehydewould be needed for the acetalization. Acetals are separated in a secondcolumn 22 and then again separated from each other and subsequentlyhydrolyzed. The acetaldehyde is refluxed. Column 22 is for thehydrolysis of polyols other than 2MD, DMD (or other polyol components)with the separation of MD and DMD from acetaldehyde by a flash unit orcolumn 23. The DMD is separated from MD in a multi-stage column 24 andthe MD and DMD are separately hydrolyzed in recovery columns 25 and 26to produce ethylene glycol and propylene glycol at high purity.

EXAMPLE 1

This Example uses a semi-batch reactor to show the feasibility of thereactive distillation scheme for separation of EG and PG from aqueoussolutions. A typical composition of a product stream from a C₅ sugaralcohol hydrogenolysis reactor was chosen for these studies. As shown inTable 2, with formaldehyde as an acetalization agent in 50% excess, 98%recovery of EG and more than 99% recovery of PG was achieved. However,formaldehyde is less desirable as an acetalization chemical aspreviously discussed.

TABLE 2 Recovery of EG and PG via semi-batch acetalization and reactivedistillation Initial Final concentration in concentration solution (wt%) (% removal in solution in parenthesis) Species (wt %) Run #1 Run #2EG  7.81 1.64 (79%)  0.16 (98%) PG 11.04 0.23 (98%) ˜0.0 (99+%) Glycerol 1.54 NA NA Xylitol  4.62 NA NA Total 25.00

EXAMPLE 2

This Example shows the system used for separation and quantification ofthe products.

Distillation column: As shown in FIG. 4, the column 40 consisted of a 2″(5.08 cm) diameter×7 ft (213.4 cm) tall Pyrex tube in 3′ (36.25 cm) and4′ (121.92 cm) sections. The column 40 contained Katamax structuredpacking 41 from Koch-Glitsch, Ltd. (Wichita, Kans.); up to 15 elementswere placed into the column for a total packed height of 82″ (208.28cm). The catalyst, a 1 mm ion exchange resin in acid form (Amberlyst15™, Rhome & Haas), was contained in folded pouches inside each element.The column 40 was wrapped with heating tape (not shown) in two-footsections; the temperature of each heating tape was controlled, with asurface thermocouple and Omega controller to near the internal column 40temperature to minimize heat losses. At the bottom of the columnreboiler 42 consisted of a 1000 ml round-bottom flask 43 held in aheating mantle 44; the reboiler 42 had an overflow level control tomaintain a constant inventory in the reboiler flask. A glass refluxsplitter 45 with a reflux condenser made up the top of the column 40;electronic timers control the reflux ratio at the desired value. Thecondenser was cooled by a 40 wt % solution of ethylene glycol circulatedthrough a chiller to allow condenser temperatures as low as −20° C. Twofeed pumps F1 and F2 dispense feed solutions to the column at acontrolled rate from 1 to 200 ml/min. The column itself had 14 portsthat allowed temperature measurements, introduction of feed, or samplewithdrawal. The column had sections 12, 13 and 15 as shown in FIG. 1.

Analytical Method

Analysis techniques used liquid chromatography (HPLC) and gaschromatography (GC) for analysis of glycols and acetals. An improved GCmethod was the use of a slightly polar Porapak R packed column (6′×⅛″OD) that enabled the separation of glycols, acetals, water andacetaldehyde in one injection. The analysis was conducted in a Varian3700 gas chromatograph equipped with a thermal conductivity detector andusing helium carrier gas at a flow rate of 0.45 cm³/s. Columntemperature was initially maintained at 140° C. for 2 minutes and thenincreased to 230° C. at a ramp rate of 45° C./min. The injector anddetector block temperatures were maintained at 230 and 250° C.,respectively. This method allows separation and quantification of water,acetaldehyde, 2MD, 24DMD, EG and PG. HPLC was used for analysis ofglycols and other organic compounds produced in the bottom streams ofthe distillation. A Bio-Rad HPX-87H column with 0.005 M H₂SO₄ as amobile phase, 50° C. column temperature, and refractive index detectionwas used.

EXAMPLE 3

Reaction equilibrium: Batch studies were carried out at several reactiontemperatures to determine the reaction equilibrium for glycols recoveryusing acetaldehyde.

EG+acetaldehyde=2-methyl-1,3-dioxolane (2MD)+water

PG+acetaldehyde=2,4-dimethyl-1,3-dioxolane (24DMD)+water

The equilibrium constant for these reactions are given as

K _(e) =C _(2MD) C _(H2O) /C _(EG) C _(acetaldehyde)

K _(e) =C _(24DMD) C _(H2O) /C _(PG) C _(acetaldehyde)

where C is the concentration of the species in the reaction solution.The results of these experiments are given in Table 3 as a function oftemperature. Also given in Table 3 is the equilibrium constant for thereaction of PG with acetone, taken from a paper by Chopade (Chopade, S.,Reactive and Functional Polymers, vol. 42, p201 (1999)).

TABLE 3 Reaction equilibrium constants for acetal formationEG-Acetaldehyde PC-Acetaldehyde PG-Acetone T(° C.) K_(e) T(° C.) K_(e)T(° C.) K_(e) 25 6.1 25 18.2 30 0.3 44 4.9 40 17 40 0.24 82 3.8 59 13.882 8.4

EXAMPLE 4

This Example describes VLLE data for two acetals and water, and vaporpressure data for the acetals. These thermodynamic data are importantfor determining the efficacy of the process.

Thermodynamic data for acetals and acetal/water mixturesVapor-Liquid-Liquid Equilibrium (VLLE): An Othmer still, traditionallyused for the collection of vapor-liquid equilibrium data (Othmer, D.,Ind. & Eng. Chem. 20 743 (1928)), was used to facilitate the collectionof VLLE data for the systems 2MD-water and 24DMD-water. The pure acetalsused in the experiments were prepared by batch processing as follows:excess aldehyde was added to EG or PG, stirred in the presence of ionexchange resin for several hours, and then distilled to recover theacetal-water azeotrope. This azeotrope was then dried using molecularsieves to remove all water present. To determine VLLE data, specifiedquantities of acetal and water were placed in the Othmer still andbrought to reflux. After steady state was reached, as evidenced bycontinuous reflux of the condensed vapor back into the still pot and aconstant liquid temperature, samples of each liquid phase and condensedvapor were taken and analyzed as described above. The diagrams are thusgenerated by changing the mole fraction in the initial charge to thestill across the entire composition range from zero to one.

The T-x-x-y diagrams for 2MD-water and 24DMD-water are given in FIGS. 5and 6, respectively; these diagrams contain both the experimental dataand the fit of the data as described below. Water and acetals are onlypartially miscible, so there are regions where two liquid phases arepresent (thus vapor-liquid-liquid information is required). In additionto the presence of two liquid phases, both acetals form minimum-boilingazeotropes with water. It is this minimum-boiling azeotrope that makesthe technology especially attractive, as the lower boiling point atwhich the acetal can be recovered is advantageous.

Although the phase equilibrium is somewhat complex, it is possible totake advantage of these complexities to induce more efficientseparations than would otherwise be possible. The VLLE data areimportant for understanding experiments, conducting process design, andmodeling and conducting economic assessment of the technology.

The VLLE data for the acetal-water systems were fit to the UNIQUACequation of state in order to facilitate more practical use of the data.The outputs from data regression include the estimated vapor-liquid datausing UNIQUAC and the UNIQUAC binary interaction parameters. Theestimated vapor-liquid data must be a close fit to the experimentalvapor-liquid data in order to be of any use. FIGS. 5 and 6 show thecomparison between the estimated and actual experimental equilibriumdata for the systems of 2MD-water and 24DMD-water. Table 4 gives theUNIQUAC binary interaction parameters for each system.

TABLE 4 UNIQUAC Binary Interaction Parameters Component I DMD 2MD DMDComponent J Water Water Actaldehyde Temperature K K K A_(IJ) −14.6864.31 −0.1225 A_(JI) −56.15 −80.00 −0.1708 B_(IJ) 3871.28 2305.94244.763 B_(JI) −3981.73 −1339.78 450.817 C_(IJ) −2.69 −15.769 −0.0280C_(JI) 16.97 18.214 −0.0746 D_(IJ) 0.0495 0.0567 0.000045 D_(JI) −0.0904−0.0645 −0.004122

Vapor pressure data: As with the acetal-water VLLE data, vapor pressuredata are needed to assess the separation of the acetals and forsimulation studies (such as the UNIQUAC fitting of VLLE data describedabove). Vapor pressure data for pure acetals were collected in a closedpressure vessel. Initially, a small quantity of pure acetal was placedin the vessel and the vessel was placed in an ice bath. When it wascooled, vacuum was applied to remove air, but not strongly enough toboil off the acetal. The initial pressure was noted, and then the closedassembly was put in a constant temperature water bath and allowed toequilibrate. The final pressure was recorded; the difference between theinitial and final pressure is the vapor pressure at that temperature.The experiment was repeated at a number of temperatures.

The experimental vapor pressure data are shown in FIG. 7. The constantsin Antoine's equation, which is the standard form used to characterizevapor pressure data, were calculated from the above experimental data.Further, the heat of evaporation was calculated from the vapor pressuredata using the Clausius-Clapeyron equation. The Antoine's constants,predicted boiling point, and predicted heat of evaporation are given inTable 5. The predicted values agree very closely with the experimentalvalues, thus verifying the accuracy of the experimental data.

TABLE 5 Predicted Antoine's constants and heat of vaporization of 2MDand 2,4-DMD a) Antoine's constant Dioxolane A B C Range (C) MD −19.675115909.9 −688.602 25-80 2,4-DMD −4.98443 3238.18 −369.988 25-90 b)Boiling points Predicted Boiling Point (C.) Dioxolane Boiling point (C.)Antoine's eqn. Exponential graph MD 82-83 83.8 83.6 2,4-DMD 92 91.2691.8 c) Predicted heat of vaporization by Clausius-Clapeyron equationsExpt NIST Std Source calculated (Reported) (Reported) Acetal −Hv/R (K) BHv(KJ/mol) KJ/mol KJ/mol 2MD −4230.60 89.033 35.17322 35 34.32 2,4-DMD−3684.16 62.33 30.63007 NA NA

Separation of Acetals by Conventional Distillation

The ratio of the vapor pressures of the two acetals (from FIG. 7) isequal to the relative volatility of the 2MD to 24DMD if mixtures of thetwo are considered ideal. The plot of temperature versus relativevolatility is shown in FIG. 8. At the temperature range over whichseparation of the two species would take place at atmospheric pressure(80-90° C.), the value of the relative volatility of 2MD and 24DMD isabout 1.3. Thus, separation of 2MD from 24DMD is possible by fractionaldistillation.

An experiment to illustrate the separation of the two acetals wascarried out in a small distillation column. The column consisted of a1¼″ diameter×5 ft tall Pyrex tube packed with wire mesh packing similarto FIG. 4. The column is wrapped with heating tape and the temperatureof heating tape is controlled (with a surface thermocouple and Omegacontroller) to near the internal column temperature to minimize heatlosses. The reboiler consists of a 500 ml roundbottom flask held in aheating mantle; the reboiler has an overflow level control to maintain aconstant inventory in the reboiler flask. A glass reflux splitter with areflux condenser makes up the top of the column; electronic timerscontrol the reflux ratio at the desired value. The condenser is cooledby a 40 wt % solution of ethylene glycol circulated through a chiller toallow condenser temperatures as low as −20° C. The feed pumps dispensefeed solutions to the middle of column at a controlled rate from 1 to200 ml/min. The column has 5 ports that allow temperature measurements,introduction of feed, or sample withdrawal.

The experiment was conducted by placing an equimolar mixture of 2MD and24DMD into the reboiler and then bringing the column to temperature attotal reflux. At this condition, 99+% pure 2MD was obtained at the topof the column and 99+% 24DMD was obtained at the bottom of the column,respectively. This clearly indicated the feasibility of separation ofthese acetals in a single column.

EXAMPLE 5

Continuous reactive distillation: FIG. 9 shows the two columns 100 and101 used in Example 2. Column 100 is for the reactive distillation andcolumn 101 is for the separation of the acetals from the acetaldehyde.Vessels 102 and 103 are reflux condensers and vessel 104 is the reboilerfor vessel 100. Vessel 105 is a reboiler for vessel 101. This systemallows for recycling acetaldehyde while maintaining favorableacetaldehyde and glycol molar ratios in column 100. Because acetaldehydeis so volatile and somewhat difficult to handle, a two-column system 100and 101 was used for acetaldehyde-glycol studies to recycle acetaldehydethrough the reactive distillation column 100. This system, shownschematically in FIG. 9, allows maintenance of high ratios ofacetaldehyde to glycol (up to 10:1) in the column 100 while notconsuming large quantities of acetaldehyde. Experiments were conductedto demonstrate acetal recovery in a reactive distillation columndescribed in Example 2. The system PG-acetone was used initially forshakedown and to develop a familiarity with column 100 operation,because acetone has a boiling point of 58° C. (as opposed toacetaldehyde, which boils at 21° C.) and is easily handled at roomtemperature.

Results of the continuous reactive distillation experiments are given inTable 6.

TABLE 6 Results of continuous reactive distillation experiments GlycolGlycol feed Glycol Reflux Height of concen- Glycol concen- solutionratio in catalyst tration in conversion tration feed rate column sectionbottoms to acetal System (Wt %) (g/min) (L/D) (in.) (wt %) (%)PG-acetone 100 3.6 0.25 39 83 83 PG-acetone 75 3.6 0.25 39 63 47EG-acetaldehyde 50 3.6 1 28 16 55 EG-acetaldehyde 50 6.0 1 28 29 42PG-acetaldehyde 50 3.6 1 28 0.6 99 PG-acctaldehyde 25 3.6 0.75 28 1.4 94

The column operated as expected in initial shakedown runs usingPG-acetone, but the formation of the acetal,2,2,4-trimethyl-1,3-dioxolane, was substantial only at highconcentrations of PG (50%-100%) in the feed solution. These initial runsdemonstrate that the ion exchange resin is active for acetal formation.

The experiments with EG-acetaldehyde and PG-acetaldehyde clearlydemonstrate the feasibility of glycol recovery via acetal formation. Inthe run using 50% PG feed solution in water, 99% of PG is removed fromthe aqueous feed stream, leaving a bottoms product of essentially purewater. The recovery of EG is lower than for PG, corresponding to thelower reaction equilibrium conversion of EG to 2MD. The potential forincreasing all recoveries is excellent, as these experiments werecarried out with only 28″ (71.12 cm) of catalyst section and moderateacetaldehyde recycle rates. Longer catalyst section would allowdemonstration of nearly complete EG recovery.

The results compiled demonstrate the reactive distillation and recoveryof polyols from aqueous solution.

EXAMPLE 6

This example shows a mixed feed solution of sorbitol, glycerol, EG, andPG. Only the acetals of EG and PG are removed in the top of the column,along with acetaldehyde. The bottoms consist of unreacted EG, PG,glycerol, sorbitol, and acetals of glycerol and sorbitol. The resultsare shown in Table 7.

TABLE 7 Acetalization of simulated solution *Table shows only topcomposition in mol % Feed: 15% PG + 7% EG + 5% Glycerol + 5% Sorbitol(wt %) Molar Feed Feed Feed rate, feed ratio, Conv % Bottom PositionPosition AcH Feed: Reflux PG Top composition, mol % Composition,Temperature profile, C Run AcH (Solution) Mol/min AcH ratio (EG) H2O AcHMD DMD mol % Top Middle Bottom 1. F2 F1 0.101 1/6 1:4 52.28 25.94 61.232.33 10.50 Mixture of 48 68-101 101- 18.47 unreacted feed 103 2. F2 F10.119 1/5 1.4 34.20 49.14 42.24 1.72 6.89 and their 65 85-101 101- 11.94acetals of high 103 boiling points (PG: 0.42%) (EG: 1.2%) ColumnSpecifications: Enrichment section 15 cm, Reaction zone (with catalyst)140 cm, Stripping section 45 cm Feed position: F1, 15 cm from top; F2,45 cm from top, and F, 45 cm from bottom. (AcH: acetaldehyde, MD:2-methyl dioxolane, DMD: 24 dimethyl 1,3 dioxolane; PG: Propyleneglycol, EG: Ethylene glycol, H2O: water) Feed rate: 0.02 mol/min *At topthere are only MD and DMD acetals

EXAMPLE 7

This Example shows the results with a feed solution containing a mixtureof EG, PG and other polyols. There was no hydrolysis in section 14 ofFIG. 1. The results are shown in the acetals of EG and PG produced.Small quantities of 4-ethyl-2-methyl-1,3-dioxolane (EMD), the acetal of1,2-butanediol, were present at the top of the distillation column. Theconcentration of EMD will be reduced in a taller, commercial-scalesystem. No glycerol or sorbitol cyclic acetals were found in the PG orEG produced. The results are shown in Table 8.

TABLE 8 ACETALIZATION OF MIXED SOLUTION OF POLYOLS Feed: 15% PG + 7%EG + 5% Glycerol + 5% Sorbitol + 2% 1-2 Butanediol (all in wt %) BottomMolar Top composition, mol % composition, Feed Feed feed From Column 11mol % Posi- Feed rate, ratio, Re- Conv % EMD From Temperature profile, Ction Position AcH Feed: flux PG (Acetal of 2- Column In Column 11 RunAcH (Solution) Mol/min AcH ratio (EG) H2O AcH MD DMD butanediol) 11 TopMiddle Bottom 1. F2 F1 0.119 1/6 1:4 53.27 29.31 54.97 3.63 12.04 0.023Mixture of 52 68-101 101-103 25.39 unreacted 2. F2 F1 0.101 1/5 1.427.53 49.14 42.24 1.72 6.89 0.45 feed and 70 82-101 101-103 10.85 theiracetals 3 F2 F1 0.082 1/4 1:4 57.78 21.03 54.96 5.67 18.28 0.03 of high44 65-101 101-103 28.28 boiling 5. F2 F1 0.06 1/3 1:4 54.47 15.32 66.073.5 12.54 2.4 points 50 66-101 101-103 24.61 4. F2 F1 0.119 1/6 1:469.22 14.1 67.58 8.95 29.21 0.09 44 62-101 101-103 31.44 (12) (13) (15)Column Specifications: Enrichment section 15 cm, Reaction zone (withcatalyst) 140 cm, Stripping section 45 cm; Length 200 cm Feed position:F1, 15 cm from top; F2 45 cm from top, and F, 45 cm from bottom. (AcH:acetaldehyde, MD: 2-methyl dioxolane, DMD: 24 dimethyl 1,3 dioxolane;EMD: 4-ethyl-2-methyl-1,3-dioxolane PG: Propylene glycol, EG: Ethyleneglycol, H2O: water) (No glycerol or sorbitol cyclic acetals in product)Feed rate: 0.02 mol/min

EXAMPLE 8

Example 8 shows the fractional distillation of DMD and MD mixtures withand without water. The distillate column was 1½″ (3.8 cm) in diameterand 5 ft (152.4 cm) in length with wire mesh packings. The results inTables 9 and 10 show that such separations are feasible.

TABLE 9 A) Distillation of mixture of pure components Distillate BottomTemperature composition, composition, profile, Reflux mol % mol % C Runratio MD DMD MD DMD Top Bottom 1 Total 81.56 18.44 0.0 99.999 83 102Reflux 2 Total 91.7 8.3 0.0 99.999 82 97 Reflux

TABLE 10 B) Distillation in presence of water Distillate composition,Bottom composition, Temperature Reflux mol % mol % profile, C Run ratioMD DMD Water AcH MD DMD Water AcH EG PG Top Bottom 1 Total 57.17 3.0139.36 0.00 17.80 82.19 0.0 — — — 75 92 Reflux 2 Total 37.81 17.43 33.2411.5 0.0 87.17 0.0 0.0 6.3 6.4 82 104 Reflux (At high temperaturehydrolysis of acetal takes place)

EXAMPLE 9

Acetal Hydrolysis via Reactive Distillation

Having demonstrated that the acetals of EG and PG can be formed andrecovered via reactive distillation using an ion exchange resin in acidform (Amberlyst 15), the hydrolysis of acetals in the reactivedistillation column to obtain high purity propylene and ethylene glycolwas examined. The emphasis was primarily to have high purity propyleneglycol, specifically with a very low (ppm) level of acetaldehydeimpurity. As part of this effort, process simulation software was usedwith the VLLE data to help design the lab experiments and verify thepotential of obtaining high purity PG.

Hydrolysis of 2,4DMD: Initial experiments were carried out with a 100 cmreaction zone (packing with catalyst) and 200 cm of total structuredpacking (Katamax structured packing, Koch-Glitsch, Ltd., Wichita,Kans.). Because of substandard performance with 100 cm of reaction zone,later experiments were conducted with a 140 cm reaction zone and 200 cmtotal packing. The catalyst, 1 mm ion exchange resin beads in acid form(Amberlyst 15), is contained in folded pouches inside each element ofthe reaction zone. The details of the packing are mentioned in theprevious Examples.

The column was operated under steady state at a variety of refluxratios, temperature profiles, and water:acetal feed ratios. Theexperimental results are tabulated in Table 11.

TABLE 11 Hydrolysis of 2,4 Dimethyl dioxolane Feed Feed Feed rate, Molarfeed Distillate composition, Bottom composition, Temperature PositionPosition DMD ratio, Reflux Conv % mol % mol % profile, C Run H2O DMDmol/hr DMD:H2O ratio (DMD) H2O AcH DMD H2O DMD PG Top Middle Bottom 1 F2F 1.99 1/2 1:2 64.29 15.59 67.87 16.13 69.61 0.00 30.38 42 82-98  98-1032 F2 F 1.99 1/2 1:2 72.89 40.51 47.09 12.38 77.70 0.00 21.17 64 86-99100-106 3 F1 F 1.99 1/2 1:2 77.22 17.02 65.59 16.98 79.24 0.00 20.75 5684-98  99-105 4 F1 F 1.99 1/2 1:4 80.13 17.06 66.28 16.65 78.41 0.0021.58 54 84-98  99-105 5 F1 F 1.99 1/1.2 1:4 81.7 13.12 73.40 13.4665.12 0.00 34.75 54 84-98  99-106 6 F1 F 1.99 1/1.5 1:4 81.11 23.7863.07 13.13 69.00 0.00 30.99 54 84-98  99-106 7 F1 F 1.99 1/3 1:4 79.5420.88 66.46 12.65 80.09 0.00 19.05 54 84-98  99-106 ColumnSpecifications: Enrichment section 15 cm, Reaction zone (with catalyst)140 cm, Stripping section 45 cm Feed position: F1, 15 cm from top; F2 45cm from top, and F, 45 cm from bottom.

With 100 cm of reaction zone, conversion of 24DMD to PG of about 75% wasobtained. With the longer reaction zone of 140 cm and a shorterrectifying section, which allows more residence time in the catalyticsection, up to 80% DMD conversion was achieved. Process simulationpredicts and experiments verify that a water:acetal ratio of 1.2 to 2 issufficient; higher water:acetal ratios do not further improve columnperformance. Most importantly, the PG product coming from the bottom ofthe column is very pure as seen in Table 11.

TABLE 12 Hydrolysis of 2 Methyl dioxolane Feed Feed Feed rate, MolarDistillate composition, Bottom composi- Temperature Position Position MDfeed ratio, Reflux Conv % mol % tion, mol % profile, C Run H2O MD mol/hrMD:H2O ratio (MD) H2O AcH MD H2O MD EG Top Middle Bottom 1 F1 F 2.67 1/21:4 80.0 21.7 73.90 4.30 71.04 0.00 28.95 54  98-102 104-108 2 F1 F 2.671/3.8 1:4 81.5 15.9 78.20 5.92 84.14 0.00 15.85 48 96-99 104-108 3 F1 F2.67 1/2 1:4 91.7 20.6 65.50 13.80 74.12 0.00 27.87 40 96-99 104-108 4F1 F 2.67 1/3 1:4 96.0 17.6 77.95 4.47 77.34 0.00 22.65 40  96-100104-110 5 F1 F 2.67 1/4 1:4 93.4 18.06 75.66 6.27 82.43 0.01 16.35 44 96-101 104-108 6 F1 F 2.67 1/1.2 1:4 88.3 19.5 75.50 4.99 65.38 0.0034.61 42  96-101 105-120 Column Specifications: Enrichment section 15cm, Reaction zone (with catalyst) 140 cm, Stripping section 45 cm Feedposition: F1, 15 cm from top; and F, 45 cm from bottom.

Hydrolysis of 2MD: In experiments parallel to those described above for24DMD, hydrolysis of 2MD was studied. The experimental results aresummarized in Table 12, and show that higher conversions of 2MD to EG,up to 95%, can be achieved than for 24DMD to PG.

TABLE 13 Hydrolysis of mixed acetal (2MD and 2,4 DMD) Molar Feed feedFeed rate, ratio, Conv Feed Point mol/h DMD + Re- % Top composition,Bottom composition, Temperature Point Ace- DMD + MD: flux DMD mol % mol% profile, C Run (H2O) tal MD H2O ratio (MD) H2O AcH MD DMD H2O MD DMDEG PG Top Middle Bottom 1 F1 F 2.24 1/2.5 1:4 75.92 49.82 31.03 2.216.96 77.5 0.0 0.0 9.51 12.97 60 98-102 100-112 91.03 2 F1 F 2.24 1/2.51:4 83.7 10.35 78.17 1.37 10.10 82.65 0.0 0.001 6.86 10.48 43 97-98  99-107 89.9 3 F1 F 2.24 1/2.5 1:4 86.69 13.32 76.14 1.43 9.38 71.36 0.00.0 11.15 17.48 48 97-99  100-110 90.48 5 F1 F 2.24 1/4.1 1:4 87.8 9.582.35 0.5 7.6 83.15 0.0 0.0 6.5 10.25 43 97-98   99-107 92.4 4 F1 F 2.241/1.2 1:4 78.06 17.42 69.62 1.34 11.55 72.01 0.0 0.0 11.33 16.65 5098-101 101-120 86.96 Feed: 62% mole DMD and 38% mole MD ColumnSpecifications: Enrichment section 15 cm, Reaction zone (with catalyst)140 cm, Stripping section 45 cm Feed position: F1, 15 cm from top; F2 45cm from top, and F, 45 cm from bottom.

This is because the reaction equilibrium for hydrolysis of the acetal ofEG is more favorable than that for hydrolysis of the acetal of PG. Thereaction equilibrium constants are tabulated in Table 3.

The conversion of acetal to glycol in hydrolysis is limited in theseexamples by the height of packing available in the laboratory-scalecolumn. With a longer reactive zone, complete hydrolysis of the acetalwill take place. This is supported by the process simulations set forthhereinafter.

Alternative mixed acetal hydrolysis scheme: In an alternative scheme forthe separation and hydrolysis of the acetals formed in reactivedistillation, a mixture of both acetals are first hydrolyzed in onereactive distillation column and then the resulting mixture of PG and EGis separated in a conventional distillation column. This route canpotentially reduce the number of distillation columns required fromthree to two, with only one reactive distillation column versus two inthe original concept. This alternate route involves the separation of EGfrom PG, which is practiced commercially but is a difficult, expensiveseparation. This alternate scheme is shown as columns 29 and 30 in FIG.10.

The hydrolysis of mixed acetals (2MD and 24DMD) was carried out in thesame reactive distillation column as described above for individualacetal hydrolysis. Experiments were performed with mixtures of acetalsonly and with the acetals with water; mixture compositions were chosento simulate the products from the acetal formation column. The resultsare tabulated in Tables 13 and 14 as a function of water: acetal ratioand reflux ratio.

TABLE 14 Hydrolysis of mixed acetal (2MD and 2,4 DMD) along with waterOver all Feed Feed Feed Molar Conv % Point Point rate, mol/h feed ratio,DMD Top composition, mol % Run (H2O) Acetal DMD + MD DMD + MD:H2O Refluxratio (MD) H2O AcH MD DMD 6 F1 F 2.17 1/2.75 1:4 81.14 20.34 63.20 2.3414.12 93.68 7 F1 F 2.17 1/3.1 1:4 84.68 10.93 77.85 1.53 9.68 89.9Bottom Composition, mol % Temperature profile, C. Run H2O MD DMD EG PGTop Middle Bottom 6 81.86 0.0 0.0 8.33 9.8 44 97-99 100-110 7 80.53 0.00.0 8.5 10.88 46 96-99 100-110 Feed: 52% mole DMD and 33% mole MD + 15%mole H₂O Column Specifications: Enrichment section 15 cm, Reaction zone(with catalyst) 140 cm, Stripping section 45 cm Feed position: F1, 15 cmfrom top; F2 45 cm from top, and F, 45 cm from bottom.

Up to 95% conversion of 2MD and 85% conversion of 24DMD, values evenslightly higher than those for the individual acetals, were achieved atoptimum conditions. These results are promising and indicate that mixedacetal hydrolysis is a viable alternative to the original concept ofseparating acetals prior to hydrolysis.

Process Simulation of Hydrolysis

Computer process simulation software (Aspen Plus 10.1, Aspentec, Inc.)was used to model the proposed reactive distillation hydrolysis process.These simulations were conducted in part to help define experimentalparameters for developmental studies, and more importantly todemonstrate the behavior of commercial scale processes particularlyregarding product purities. Process simulation provides a means, basedon experimental findings and thermodynamic (e.g. VLLE) data, to predictwith significant confidence the performance of the proposed separationtechnology at the commercial level.

TABLE 15 Simulation results for single acetal (2,4 DMD) hydrolysisscheme 1 Feed 2 Feed 3 Distillate 4 Bottoms Mole Flow kmol/hr Water 60 010 1.28E−04 Acetaldehyde 0 0 50.00 5.37E−16 Propylene 0 0 2.56E−1149.99987 Glycol DMD 0 50 1.28E−04 1.43E−19 Total Flow 60 50 60 50KMOL/HR Temperature K 298.2 298.2 330.5 460.9 Number of Stages 20 RefluxRatio 3 Boilup Ratio 7 Water Feed Location 2 DMD Feed Location 7 HETP(m) 0.5 Reaction Zone Stage 5-15 Packed Zone Stage 5-15 Packing KerapakPacking Height (m) 5.5 Column Diameter (m) 1.78

The simulation results of 24DMD hydrolysis to PG, corresponding tocolumn 26 in FIG. 11, are given in Table 15.

The parallel results for 2MD hydrolysis to EG, corresponding to column25 in FIG. 11, are given in Table 16.

TABLE 16 Simulation results for single acetal (2MD) hydrolysis scheme 1Feed 2 Feed 3 Distillate 4 Bottoms Mole Flow kmol/hr Water 60 0 105.57E−08 Ethylene 0 0 7.79E−17 50 Glycol Acetaldehyde 0 0 50 5.00E−292MD 0 50 5.57E−08 5.00E−29 Total Flow 60 50 60 50 KMOL/HR Temperature K298.15 298.15 330.4646 470.2331 Number of Stages 25 Reflux Ratio 3.5Packing Height (m) 8 Column Diameter (m) 1.71

The columns described in Tables 15 and 16 produce approximately 60million lb mol of propylene glycol and 50 million lb mol of ethyleneglycol per year; the column diameters are 1.78 m for the 24DMDhydrolysis column and 1.71 m for the 2MD hydrolysis column. Each columnhas been optimized to reduce the number of equilibrium stages until themaximum allowable amount of water is present in the glycol productstream in accordance with industry standards. Water:acetal feed ratioswere reduced to slightly above the stoichiometric molar ratios to reduceoperating costs of the condenser and reboiler. Inlet stream temperatureswere set at room temperature to mimic experimental conditions. Emphasiswas placed on reducing the level of acetaldehyde in the glycol productstreams to the order of part-per-million levels. It is seen that theacetaldehyde content in all of the glycol product streams is negligible,indicating that very high purity PG and EG with reasonable column sizes(˜25 stages) in a commercial process can be obtained.

The simulation results of mixed acetal hydrolysis are given in Table 17.

TABLE 17 Simulation results for mixed acetal hydrolysis scheme 1 Feed 2Feed 3 Distillate 1 4 Bottoms 1 5 Distillate 2 6 Bottoms 2 Mole Flowkmol/hr Water 83.1586 28.34952 1.70E+00 2.43E−04 2.43E−04 3.69E−35Acetaldehyde 0 0 109.8069 1.01E−14 0 0 Propylene Glycol 0 0 5.30E−1168.22761 68.22689 7.13E−04 Ethylene Glycol 0.00E+00 0 6.02E−15 41.57933.27E−04 41.57897 DMD 68.22785 0 2.43E−04 2.09E−14 0 0 2MD 41.5793 01.21E−08 1.10E−28 0 0 Total Flow KMOL/HR 192.9658 28.34952 111.5081109.8072 68.22747 41.57969 Temperature K. 3.51E+02 3.48E+02 295.39564.64E+02 4.61E+02 4.70E+02 Column 1 Column 2 Number of Stages 30 123Reflux Ratio 5 12 Boilup Ratio 4.51 21.8 Water Feed Location 2 — MixedFeed Location 12 51 HETP (m) 0.5 — Reaction Zone Stage 2-15 — PackedZone Stage 2-20 0.609 m/stg Packing/Trays Kerapak Sieve Packing/ColHeight (m) 9.5 74.9 Column Diameter (m) 2.14 2.8

This hydrolysis column was optimized in the same manner as the singleacetal hydrolysis. It also produces approximately 60 million lb mol ofpropylene glycol and 50 million lb mol of ethylene glycol per year. Thecolumn diameter of the mixed acetal hydrolysis is 2.14 m, somewhatlarger than the single acetal hydrolysis columns. The second column toseparate EG and PG via conventional distillation is very large. Again, anegligible quantity of acetaldehyde is present in the product mixedglycol stream, indicating that pure EG and PG can be produced in acommercial-scale process.

Summary

The combined results of experimental findings and simulation studiesshow the feasibility of reactive distillation for polyols recovery fromaqueous solution. In particular, two viable scenarios are presented herefor the separation and hydrolysis of acetals produced in reactivedistillation to pure PG and EG. Experimental findings are in accordancewith thermodynamic and reaction data, but experimental conversion andproduct purities are limited by the size (particularly the height) ofthe laboratory-scale equipment. Simulation studies, based onexperimental data, demonstrate with a high degree of confidence that therequired product purities and recoveries can be achieved incommercial-scale equipment.

It is intended that the foregoing description be only illustrative ofthe present invention and that the present invention be limited only bythe hereinafter appended claims.

We claim:
 1. A continuous process for preparing at least one cyclicacetal from an aqueous solution having at least one polyol and at leastone other organic compound which forms another cyclic acetal by-productin the process which comprises: (a) reacting in a combination reactionand distillation vessel a reaction mixture of the polyol, organiccompound, and an aldehyde or ketone containing 1 to 4 carbon atoms inthe aqueous solution in the presence of an acid catalyst, wherein thereaction mixture is introduced into the reaction vessel containing thecatalyst with a molar excess of the aldehyde or ketone over the polyolto produce the cyclic acetal and cyclic acetal by-product in the aqueoussolution; and (b) separating at least one cyclic acetal from thereaction mixture containing the at least one cyclic acetal by-product bydistillation.
 2. The process of claim 1 wherein there is more than onepolyol in the reaction mixture which is reacted.
 3. The process of claim1 wherein the reaction mixture contains ethylene glycol and propyleneglycol as polyols which react with the aldehyde in the mixture to formthe cyclic acetal.
 4. The process of any one of claims 1, 2 or 3 whereinthe reaction mixture contains at least one of glycerol, sorbitol, and C4diols and triols as the other organic compound which react with thealdehyde to form additional cyclic acetals which have a higher boilingpoint than the cyclic acetal distilled from the reaction mixture.
 5. Theprocess of claims 1, 2 or 3 wherein in addition the excess aldehyde orketone is recycled to step (a).
 6. The process of any one of claims 1, 2or 3 wherein acid catalyst in step (a) is an acidic resin.
 7. Theprocess of any one of claims 1, 2 or 3 wherein the aldehyde in step (a)is acetaldehyde.
 8. The process of claims 1, 2 or 3 wherein the cyclicacetal and the aldehyde or ketone are distilled from the reactionmixture.
 9. The process of claims 1, 2 or 3 wherein the reaction mixturecontains cyclic acetals of at least two polyols which are separated fromthe reaction mixture together, then separated from each other and thenhydrolyzed separately to the polyols.
 10. The process of any one ofclaims 1, 2 or 3 wherein the reaction mixture is at a temperature, lessthan the boiling point of the reaction mixture, at which the aldehyde orketone is distilled from the reaction mixture.